The present invention involves measuring pressure, and particularly, to transmission of signals representative of two variables through a single analog-to-digital converter with minimum loss of information and minimum aliasing errors. More particularly, the present invention relates to correcting the output signal of a measurement circuit for errors in a differential pressure signal due to changes in line pressure or temperature on a differential pressure sensor, and to correcting the output signal for errors in a line or static pressure signal due to changes in temperature.
Capacitive differential pressure sensors often include a sensor housing having an inner chamber divided into two cavities by a deflectable diaphragm. A first pressure is provided to the first cavity, while a second pressure is provided to the second cavity. Difference between the first and second pressures causes the diaphragm to deflect, the amount of deflection being based on the amount of the difference in pressure.
The diaphragm typically includes a conductive portion separated from, and aligned with, conductive portions on the inner walls of the cavities to form first and second variable capacitors within the first and second cavities, respectively. As the diaphragm deflects due to differential pressure, the capacitive values of the two variable capacitors change. The pressure sensor is connected to a measurement circuit to provide an output signal representative of the capacitive values of the variable capacitors. The output signal provides a measurement of the differential pressure.
However, problems can arise due to non-linearities in the capacitive pressure sensor. For example, stray capacitances in the system can cause non-linearities which must be compensated.
Also, errors can result due to changes in line pressure. Line pressure, also commonly referred to as static pressure, can be defined in several ways. To illustrate the different definitions of line pressure, assume the first and second pressures provided to the first and second cavities of the capacitive pressure sensor have values of 2990 psi (P.sub.L) and 3000 psi (P.sub.H) creating a differential pressure of 10 psi (3000 psi-2990 psi). By one definition, line pressure is defined as the average of P.sub.H and P.sub.L, or 2995 psi in the example. Other definitions define line pressure as simply P.sub.H or P.sub.L, alone. Regardless of which definition is used for line pressure, errors in the output signal of the pressure sensor can result based on variations in line pressure.
The effects of variations in line pressure on a capacitive differential pressure sensor may be illustrated with the following examples. Where P.sub.H =3000 psi and P.sub.L =2990 psi, the differential pressure is 10 psi and the line pressure is 2995 psi (using the average of P.sub.H and P.sub.L as the measurement for line pressure). However, where P.sub.H =10 psi and P.sub.L =0 psi, the differential pressure is still 10 psi but the line pressure is 5 psi. Due to certain stresses placed on the housing of the pressure sensor, the output signal of a typical differential pressure sensor may vary 1% per 1000 psi variation in line pressure. Thus, with the examples given above, the output signal from the differential pressure may vary significantly with changes in line pressure. It is desirable to measure differential pressure and provide an output signal which is unaffected by variations in line pressure.
The Frick U.S. Pat. No. 4,370,890, issued on Feb. 1, 1983 and assigned to the same assignee as the present invention, discloses a mechanical configuration for a capacitive differential pressure sensor which attempts to compensate for unwanted mechanical stresses on the capacitive pressure sensor housing due to variations in line pressure. The Frick configuration reduces variations in the output signal of the differential pressure sensor due to variations in line pressure. However, there is a continuing need for correction techniques which correct for variations in the output signal due to variations in line pressure and which can be adjusted by electrical rather than mechanical means.
The Frick U.S. pending application Ser. No. 7-667,320, filed Mar. 8, 1991 and assigned to the same assignee as the present invention, describes the use of fixed compensation capacitors arranged with the variable capacitors of the differential sensor so that the currents through compensation capacitors subtract from the currents through variable capacitors. The capacitance values of the compensation capacitors are chosen to compensate the output of the circuit for zero and span errors caused by variations in line pressure. The use of fixed compensation capacitors is limited to an expected operating range for the sensor and may not be adequate for all conditions. Hence, there remains a need for improved correction techniques.
Another type of pressure sensor is a piezoresistive bridge sensor typically employing a bridge network of four piezoresistive elements formed on a single silicon wafer. The piezoresistive elements are arranged such that pressure applied to a diaphragm in the wafer unbalances the resistive values of the bridge. The two pressures P.sub.1 and P.sub.2 affect opposite sides of the diaphragm to apply differential pressure to the piezoresistive elements. Changes in differential pressure alters the impedance of two diametrically opposed piezoresistive elements of the bridge in one manner and alters the impedance of the other two diametrically opposed piezoresistive elements in an opposite manner (such as altering the impedances of piezoresistive elements R.sub.5 and R.sub.6 in FIG. 4 positively and altering the impedances of piezoresistive elements R.sub.7 and R.sub.e negatively). The resulting output signal from the bridge is representative of differential pressure.
In the case of a piezoresistive bridge for measuring line pressure (gauge or absolute pressure), changes in pressure applied to the wafer will typically alter the impedances of two opposite piezoresistive elements positively, and will alter the impedances of the other two opposite piezoresistive elements negatively. The amount of change of impedance of each piezoresistive element is representative of line pressure.
It will be appreciated by those skilled in the art that the piezoresistive bridges may be half-bridges employing only two piezoresistive elements. The piezoresistive elements of the differential pressure bridge would be responsive to differential pressure to change the impedance of one piezoresistive element positively and to change the impedance of the other piezoresistive element negatively. The piezoresistive elements of the line pressure bridge would be responsive to line pressure to change the impedance of one piezoresistive element positively and to change the impedance of the other piezoresistive element negatively. Most present piezoresistive bridges employ four piezoresistive elements for economic reasons.
Differential pressure sensing piezoresistive bridges are responsive in to changes differential pressure as well as line pressure and temperature. Line pressure sensing piezoresistive bridges are responsive to changes in line pressure and temperature. This is the result of undesirable mechanical stresses imposed on the piezoresistive elements on the wafer due to variations in line pressure and temperature and the result of undesirable changes in resistivity of the piezoresistive elements due to variations in temperature. These stresses and changes in resistivity adversely affect output signals from the bridge. Consequently, it has been common to provide a separate temperature sensor with a line pressure bridge, and a separate line pressure bridge and a temperature sensor with a differential pressure bridge. In the case of a differential pressure bridge, the output of the bridge was processed with the outputs of the temperature sensor and line pressure bridge to calculate a corrected differential pressure. In the case of a line pressure bridge, the output of the bridge was processed with the output of the temperature sensor to calculate a corrected line pressure. Each sensor required its own dedicated analog-to-digital converter, so correction of a line pressure bridge required two converters and correction of a differential pressure bridge required three converters, each converter providing sensor input to a processor. To eliminate the requirement for two or three converters, multiplexing techniques were sometimes used to alternately connect the separate sensor outputs to a single converter, but multiplexing has the disadvantage that alternate connection of the several sensor information signals to the converter results in the loss of information due to aliasing errors.